Mass Extinction

Extinction is, quite literally, the end of a particular evolutionary line, the end of a species, a family, or a larger group of organisms. While it may be bad news for the victims, it's a 'natural' event in the history of life on earth. Extinctions, mostly at the level of species, have been occurring constantly at a low 'background rate', usually matched by the rate at which new species appear - with the result that biodiversity is constantly increasing.
But there have been periods in the Earth's history when biodiversity crashes. Indeed this has been a powerful force in evolution, wiping the slate clean of up to 96% of all species, and providing the survivors with a world full of opportunities into which they can diversify. These are the mass extinctions, when more than 50% of the Earth's species vanish in the geological instant of a few million years.
The cause for the Permian and cretaceous mass extinctions is still unknown by scientists, below is a list of possible causes.
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